1. Field of the Invention
The present invention relates to a liquid crystal display unit that is used as a monitor for a personal computer (hereinafter to be referred to as a PC), a word processor, etc., and further as a display for a television receiver, and a projector. More specifically, the present invention relates to a liquid crystal display unit that carries out a control for matching an image to be displayed with the characteristics of a liquid crystal panel in order to obtain an image of a satisfactory color balance.
2. Description of the Prior Art
In recent years, along with the development of a liquid crystal panel having higher resolution and fuller color range, there has been an increase in demand for liquid crystal display units that have features such as low voltage driven, thin and light weight, in the information equipment field including personal computers as main products, and in the image equipment field including television receivers and projectors as main products.
Two widely used liquid crystal display units include a twisted nematic (TN) liquid crystal and a super twisted nematic (STN) liquid crystal. The super twisted neumatic liquid crystal has a transmission characteristic of improved sharpness due to its large twist angle. A driving system for the liquid crystal display units has shifted from a segment driving type, initially introduced, to a matrix driving type in order to realize a higher resolution. The matrix driving type has a pair of transparent electrodes divided into two and disposed to be mutually orthogonal with each other, each having a belt shape, with one transparent electrode working as a scanning electrode and the other as a signal electrode. Points of intersection between these electrodes form pixels, and they are selectively applied with voltages to display optional image information. This matrix driving type is broadly divided into a simple matrix type and an active matrix type that uses switching elements. Particularly, an active matrix driving type liquid crystal display that uses a thin-film transistor (TFT) can obtain a high resolution and a high contrast, and therefore, has been widely distributed.
The TFT active matrix liquid crystal will be explained in detail.
FIG. 1 is a functional block diagram for explaining a liquid crystal driving system. As shown in FIG. 1, a reference number 51 denotes a signal electrode drive circuit, 52 denotes a scanning electrode drive circuit, and 5 denotes a liquid crystal display panel. The scanning electrode drive circuit 52 is constructed of a shift register circuit. The output of the scanning electrode drive circuit 52 is produced from a lateral line transparent electrode 54 and is applied to a gate of a TFT that is connected in parallel in a horizontal direction on the liquid crystal panel 5. The signal electrode drive circuit 51 is constructed of a shift register and a sample holding circuit. The output of the signal electrode drive circuit 51 is produced from a vertical line transparent electrode 53 and is applied to a drain or a source of a TFT that is arrayed in a perpendicular direction on the liquid crystal panel 5. When a scanning signal is added to the gates of these TFT""s, a current is conducted between the source and the drain. When an image signal is added to the source or the drain, the liquid crystal layer is charged and is applied with an electric charge. The applied electric charge is held until the next scanning signal is given. The volume of light that passes through the liquid crystal layer changes according to the voltage applied to the liquid crystal layer. Therefore, it is possible to control the optical transmission volume by the image signal voltage. In other words, the scanning electrode drive circuit 52 turns ON the TFT in the horizontal direction together, and during this period, the signal electrode drive circuit 51 writes image information of one line component into pixels of each intersection point. This is sequentially scanned in the vertical direction to thereby display the image information.
The development of color display technology for the liquid crystal display panel is also progressing with the development of the high-resolution technology. As general methods of color display, there are a color filtering method that has RGB filters corresponding to each pixel disposed on the surface of the liquid crystal, and a three-panel method that provides a liquid crystal panel to each RGB image and supplies a back light or a front light of RGB to each liquid crystal panel. Both methods form a display of a color image for each of the RGB components, and adds and mixes these color components to display the color image. The color filtering method has features of compactness and lightweight, and has been widely distributed in PC monitors and liquid crystal TVs. The three-panel method has a large device scale but can obtain an image of a high resolution and a high luminance. Therefore, this method has been applied to liquid crystal projectors and the like.
Two input types of liquid crystal display units have been widely used. One is an analog interface liquid crystal display unit that inputs conventional video signals of conventional TV""s and videos, and the other is a digital interface liquid crystal display unit that is capable of directly inputting digital video data of PC""s. In recent years, digitization of video data has progressed rapidly along with the development of digital technology, due to increasing memory capacities as well as increasing processing capacity. Digital data can be more easily processed for the editing of videos like a non-linear editing than analog data. Further, digital data has no deterioration in image quality, and can be compressed at a high compression rate. Therefore, it is considered that the digitization of images will be further promoted in future. For the image digital data, various formats have been proposed because of a difference between a moving image and a still image, and a difference in compression methods. At present, digital data generally has eight bits (256 gradations) for each of R, G and B, and the data can be used to display full colors of about 1.63 million colors based on additive mixture of color stimuli.
Liquid crystal display panels as the display element have unique optical rotary dispersion characteristics. Optical rotary dispersion characteristics are a phenomenon where the optical transmittance changes depending on the wavelength of light and depending on the voltage. More specifically, a red color component (a long-wavelength area) becomes large and a blue color component (a short-wavelength area) becomes small in the optical transmittance during an application of a low voltage. Therefore, even when a gray scale is displayed, the white balance in each gradation is disturbed, and a coloring occurs according to the voltage applied. This not only causes an inconvenience in the gray scale display, but also interferes with color display performance. Particularly, there is a problem that when a gray portion such as a shade exists in the image, a color appears in this portion.
Further, in the color image display according to this color filtering method, a light from the pixel does not enter the corresponding RGB filter in an ideal manner, and the light leaks to another filter, which causes RGB crosstalk. This RGB crosstalk disturbs the color balance, and makes it impossible to reproduce a desired color.
A double-layer STN liquid crystal (DSTN) method is an example that solves the problem of the coloring attributable to the optical rotary dispersion characteristics. According to this method, two liquid crystal panels having optical rotary dispersion characteristics in opposite directions are superimposed with each other, and a coloring generated by a first-layer liquid crystal panel is canceled by a second-layer optical compensation liquid crystal panel, thereby achieving non-coloring. This method can substantially compensate for the optical rotary dispersion characteristics. However, this method also has many problems because the cost, the weight and the thickness are doubled and the manufacturing process is complex.
There has also been developed a technique for preventing the coloring problem by superimposing a phase compensation plate with a liquid crystal panel. For this phase compensation plate, there has been proposed a phase difference plate that is prepared by stretching in one axis direction a polymer film made of polyester, polyvinyl alcohol, or the like. This method can achieve a lightweight at low cost. However, it is impossible to completely match the phase-difference wavelength dispersion characteristics of the liquid crystal panel with the phase-difference wavelength dispersion characteristics of the polymer film. Therefore, it is not possible to compensate for the phase difference over the whole visible area. Further, there has also been proposed a technique having a pseudo twist structure by laminating a plurality of phase difference plates, with the optical axis of each plate shifted. However, this method has a problem of being costly and the contrast is lowered. In recent years, there has also been developed a compensation plate made of a liquid crystal polymer film of a cholesteric phase that has an inverse twisted structure. However, it is difficult to prepare a film that matches with the optical rotary dispersion characteristics of the liquid crystal panel. Therefore, there is a limit to non-coloring.
In the meantime, a technique for compensating for the coloring and the color balance by controlling and adjusting the image signal has been developed. This signal control adjustment has also been implemented in a CRT display as a conventional technique called white balance adjustment and gamma correction. In the CRT display, the spectrum characteristics of a fluorescent substance disposed on the surface of the display and the voltage luminance characteristics, that are a relationship between the drive voltage and the anode current, are compensated for. A curve that shows this relationship is as shown by a solid line in FIG. 2. This curve can be approximated by a straight line having a predetermined slope when it is expressed by logarithmic scale (gamma 2.2 curve). Therefore, it has been possible to obtain sufficient compensation by the gamma correction of a one-point bent line as shown by a broken line and by the white balance adjustment for adjusting the gain of each of the RGB signals at a constant rate, as shown in FIG. 2. This control technique has been implemented by an analog control using a transistor or a variable resistor. However, as shown in FIG. 3 the characteristic curve generated from the voltage luminance characteristics and the optical rotary dispersion characteristics of the liquid crystal panel is substantially irregular as compared with the characteristics of the CRT. Therefore, it has been difficult to carry out a sufficient compensation even if the conventional signal control technique for the CRT would be directly applied to the liquid crystal panel.
Further, in FIG. 3, the gamma correction for a liquid crystal panel by a two-point bent line as shown by a broken line is achieved as the analog control. However, according to this method, it has been difficult to carry out a fine adjustment, and there has been a limit to the correction of the characteristics of the liquid crystal. Further, considering the problem of the above-described RGB crosstalk, it has been impossible to carry out the correction by this method. Therefore, further improvements in this method are required.
As described above, in recent years, there has been an increase in demand for digital control processing that can carry out a fine control in circumstances where the digitization of image data has been widely promoted. Digital signal control techniques, include a method of correcting the RGB image data by a linear matrix conversion, a method of using a LUT (lookup table), a method of converting by approximation using a function, etc. A technique relating to the linear matrix conversion has been disclosed in Japanese Patent Application Laid-open Hei 5 No. 27711. According to Japanese Patent Application Laid-open Hei 5 No. 27711, a device is disclosed that changes a matrix coefficient according to an input luminance level of a digital signal of each of RGB colors in a liquid crystal display unit that converts a digital signal of each of RGB colors by a matrix circuit of 3xc3x973. According to Japanese Patent Application Laid-open Hei 5 No. 27711, another signal component is added to each of the RGB signals, and the chromaticity point displayed on the screen is shifted, thereby compensating the optical rotary dispersion characteristics which are particular to the liquid crystal panel.
In FIG. 4, RGB eight-bit image data are input to LUT processors 55a, 55b and 55c, respectively, and are corrected. Thereafter, the image signals are either supplied as digital data straight to a digital interface liquid crystal drive circuit 6, or are supplied as digital data to D/A converters 57a, 57b and 57c so that the data are D/A converted there and are then supplied to an analog interface liquid crystal drive circuit 58. The LUT processors 55a, 55b and 55c store data for correcting the optical rotary dispersion characteristics of the liquid crystal panel 5, and refer to the output data after the input data has been corrected. This LUT method uses a large volume of data, but can carry out a substantially finer correction than the correction carried out by the above-described approximation by a function and linear matrix conversion.
However, according to any one of the corrections carried out by the digital control processing disclosed in the above-described prior art, problems occur in each eight-bit color of RGB that is the main signal of the digital image. When a high-precision correction calculation has been carried out by the above-described digital control processing, the corrected data has information volume of eight bits or more in many cases. More specifically, as a result of a correction calculation carried out for eight-bit RBG data (100, 100, 100), for example, data is obtained as data converted into twelve-bit data of eight bits of an integer portion+four bits below a decimal point such as (100.16, 97.32, 120.64). In general, the twelve-bit data obtained by the conversion is directly D/A converted and the analog data is supplied to the analog interface liquid crystal display unit. However, a D/A converter circuit having such a large number of bits is expensive and would cause an increase in cost of the device.
Further, when the converted data is stored as eight-bit color data or is supplied to the digital interface of the liquid crystal display unit, the number below a decimal point is rounded off or is rounded to an integer. Therefore, a fine error occurs. It is known that the human visual system has a color adaptation effect that when a gray scale is colored, this colored gray scale is sensed as non-color by adapting this color to a background color when these colors (color phases) are similar color. Color contrast is characterized by, when a scale of a complementary color relationship such as red and green or blue and yellow is close to a gray scale, even a slight color in the gray scale is sensed strong, and further at a boundary portion where the scale is in contact, even a color that does not physically and optically exist is sensed. The optical rotary dispersion characteristics and the RGB crosstalk of a liquid crystal panel have a high possibility that different coloring occurs. As a result, there occurs a problem that because of the above-described color contrast characteristics of the human visual system the coloring is sensed with an emphasis, or even a color that does not physically and optically exist is sensed at a boundary portion that is in contact.
In order to solve the above problems, the present invention has been provided. It is an object of the invention to provide a liquid crystal display unit that can correct color reproduction particular to a liquid crystal panel by a digital signal control, that can process the correction in high precision, and that can reduce the sensing of a coloring of a gray scale attributable to a fine error generated by the digital signal control.
The present invention has been made in order to achieve the above object, the details of which are as follows.
According to a first aspect of the present invention, a liquid crystal display unit comprises:
a liquid crystal panel that can display a color image;
a liquid crystal drive circuit that drives the liquid crystal panel; and
a conversion controller that controls the conversion of image digital data consisting of digital signals of R, G and B respectively, and supplies the converted data to the liquid crystal drive circuit, wherein
the conversion controller comprises:
correcting means that corrects the image digital data to carry out a color reproduction that meets characteristics of the liquid crystal panel; and
color improving means that adds a fine variation to the corrected image digital data.
Further, according to a second aspect of the invention, a liquid crystal display unit according to the first aspect, further comprises:
an image data memory that stores image digital data and supplies it to the conversion controller, wherein
the liquid crystal drive circuit is driven by digital signals from the conversion controller.
Further, according to a third aspect of the invention, liquid crystal display unit according to the first aspect, further comprises:
an A/D converter that converts analog input signals of R, G and B respectively that are color image signals input from the outside of the unit into digital signals, and supplies the digital signals to the conversion controller; and
a D/A converter that converts image digital data from the conversion controller into analog signals, and supplies the analog signals to the liquid crystal drive circuit, wherein
the liquid crystal drive circuit is driven by analog signals from the D/A converter.
Further, according to a fourth aspect of the invention, a liquid crystal display unit according to the first, second or third aspect is characterized in that the correcting means carries out a correction by referring to a lookup table consisting of characteristic data of the liquid crystal panel, and each data of the lookup table that becomes the corrected data has bits of a larger number than the number of bits of the image digital data.
Further, according to a fifth aspect of the invention, a liquid crystal display unit according to the first, second or third aspect is characterized in that the correcting means carries out a correction calculation using a functional expression by approximating characteristic data of the liquid crystal panel, and the corrected data has bits of a larger number than the number of bits of the image digital data.
Further, according to a sixth aspect of the invention, a liquid crystal display unit according to the first, second or third aspect is characterized in that the correcting means carries out a correction by linearly matrix converting the image digital data using a matrix coefficient that has been calculated from characteristic data of the liquid crystal panel, and the corrected data has bits of a larger number than the number of bits of the image digital data.
Further, according to a seventh aspect of the invention, a liquid crystal display unit according to the fourth aspect is characterized in that the color improving means has a random number generator that generates a random threshold value, and changes the corrected data into an integer by the random threshold value, thereby to add a fine variation.
Further, according to an eighth aspect of the invention, a liquid crystal display unit according to the fifth aspect is characterized in that the color improving means has a random number generator that generates a random threshold value, and changes the corrected data into an integer by the random threshold value, thereby to add a fine variation.
Further, according to a ninth aspect of the invention, a liquid crystal display unit according to the sixth aspect is characterized in that the color improving means has a random number generator that generates a random threshold value, and changes the corrected data into an integer by the random threshold value, thereby to add a fine variation.
Further, according to a tenth aspect of the invention, a liquid crystal display unit according to the fourth aspect is characterized in that the color improving means stores a dither matrix pattern, and changes the corrected data into an integer by a threshold value obtained from the dither matrix pattern, thereby to add a fine variation.
Further, according to an eleventh aspect of the invention, a liquid crystal display unit according to the fifth aspect is characterized in that the color improving means stores a dither matrix pattern, and changes the corrected data into an integer by a threshold value obtained from the dither matrix pattern, thereby to add a fine variation.
Further, according to a twelfth aspect of the invention, a liquid crystal display unit according to the sixth aspect in characterized in that the color improving means stores a dither matrix pattern, and changes the corrected data into an integer by a threshold value obtained from the dither matrix pattern, thereby to add a fine variation.
Further, according to a thirteenth aspect of the invention, a liquid crystal display unit according to the tenth aspect is characterized in that the color improving means stores the dither matrix patterns separately in the image data of R, G and B respectively, and can change a variation that is added to the image data of R, G and B respectively.
Further, according to a fourteenth aspect of the invention, a liquid crystal display unit according to the eleventh aspect is characterized in that the color improving means stores the dither matrix patterns separately in the image data of R. G and B respectively, and can change a variation that is added to the image data of R, G and B respectively.
Further, according to a fifteenth aspect of the invention, a liquid crystal display unit according to the twelfth aspect is characterized in that the color improving means stores the dither matrix patterns separately in the image data of R, G and B respectively, and can change a variation that is added to the image data of R, G and B respectively.
In the present invention, the correcting means of the conversion controller can correct the color reproduction by a digital signal control according to the optical rotary dispersion characteristics that is the phenomenon which is particular to the liquid crystal panel. Further, as the color improving means adds a fine variation, the coloring of the gray that becomes the problem in the digital control processing consists of color components that are minutely different. Therefore, it is possible to reduce the emphasizing of the coloring due to the color contrast and the sensing of a color that does not physically and optically exist at the boundary portion.